The growth in use of small electrically-powered devices has increased the demand for very small metal-air electrochemical cells. Such small cells are usually disc-like or pellet-like in appearance, and are about the size of garment buttons. These cells generally have diameters ranging from less than 0.25 inch up to about 1.0 inch, and height ranging from less than 0.15 inch up to about 0.60 inch. The small size and the limited amount of electrochemically reactive material contained in these small metal-air cells result in considerable attention being directed to improving the efficiency and completeness of the power generating electrochemical reactions which occur therein.
Metal-air cells convert atmospheric oxygen to hydroxyl ions in the air cathode. The hydroxyl ions then migrate to the anode, where they cause the metal contained in the anode to oxidize. Usually the active anode material in such cells comprises zinc.
More particularly, the desired reaction in a metal-air cell air cathode involves the reduction of oxygen, the consumption of electrons, and the production of hydroxyl ions, the hydroxyl ions being able to migrate through the electrolyte toward the anode, where oxidation of zinc may occur, forming zinc oxide, and liberating electrons.
In most metal-air cells, air enters the cell through a port extending through the bottom of the cathode can. The port extends through the bottom of the cathode can, and may be immediately adjacent the cathode assembly, or may be separated from the cathode assembly by an air chamber or an air diffusion member.
In any of such arrangements, the port facilitates the movement of air through the port and into the cathode assembly. At the cathode assembly, the oxygen in the air reacts with water at the cathode assembly as a chemically reactive participant in the electrochemical reaction of the cell, and thereby forms hydroxyl ions.
In normal operation, the reaction surface of the cathode assembly is laden with electrolyte, water being a major constituent of the electrolyte. Accordingly, the water at the reaction surface of the cathode assembly has a vapor pressure, and is subject to evaporation at the reaction surface. To the extent water does evaporate at the reaction surface, moisture content of the cell is reduced, along with a corresponding reduction in efficiency of the cell. Where moisture loss is excessive, the cell may fail before the electrochemical reaction materials have been chemically used up.
A second, and undesirable function facilitated by the port in the bottom of the cathode can is that moisture e.g. evaporated from the reaction surface of the cathode assembly can escape from the cell through the port, whereby the cell dries out, and correspondingly loses effectiveness. Thus, there is a relationship between the amount of oxygen that can be made available to the cell through conventional port configurations, and the amount of moisture loss associated with such port configurations.
It is an object of this invention to provide improved cathode can structure for a metal-air electrochemical cell, the cathode can having one or more air entry ports so structured and configured, both individually and relative to each other, that the port configuration provides an improved relationship between the amount of oxygen that is available to the cathode assembly and the amount of moisture lost from the cell through the port configuration.
It is another object to provide improved cathode can structure for a metal-air electrochemical cell, wherein the sum of the open area of the port configuration is reduced while maintaining the cell limiting current.
It is still another object to provide improved cathode can structure for a metal-air electrochemical cell, the cathode can having a plurality of ports, with the port configuration structured so that, in a metal-air cell made with the cathode can, oxygen is more uniformly distributed over the cathode assembly, while minimizing the combined open area of the ports through the cathode can, and thereby reducing the amount of moisture loss through the ports.
A further object is to provide improved metal-air electrochemical cells having an increase in the ratio of the limiting current of the cell to the combined area of gaseous ingress and egress available through the port configuration.